The present invention is in the field of glass-plastic composites, and particularly relates to methods for edge-finishing glass-plastic laminated lenses which reduce the incidence of shaling fracture occuring during finishing.
It has been proposed to provide laminated articles comprising glass and plastic layers which would combine the desirable properties of both plastics and glasses, e.g., the light weight and toughness of plastics and the scratch resistance or light-responsive characteristics of glasses. For example, German Auslegeschrift No. 1,284,588 by Gliemeroth describes laminated glass-plastic articles comprising plastic and photochromic glass layers which could be used to provide optically clear glass-plastic laminates exhibiting photochromic properties.
A particularly desirable glass-plastic laminate for optical and ophthalmic applications is a laminate comprising a relatively thin sheet glass core element composed of photochromic glass which is positioned between two relatively thick plastic surface layers bonded to the front and back surfaces of the glass core. Such a laminate combines the desirable properties of very light weight and fatigue-free photochromic behavior. Laminates of this configuration may be produced by the high-temperature lamination of sheet glass and plastic members, or by casting plastic resins directly against the glass core to form the plastic surface layers of the laminate. The copending, commonly assigned application of S. T. Gulati et al., Ser. No. 018,107, filed Mar. 7, 1979, describes direct-cast laminated lenses.
One substantial problem which arises in the manufacture of a laminated glass-plastic lens blank of the kind described has been shaling fracture of the glass core member which occurs as the laminate is cooled to room temperature after processing at elevated temperatures, or as the laminate is subsequently handled. This type of failure occurs because the covering plastic surface layers, to which the glass core element is very strongly bonded, exhibit substantial shrinkage with respect to the glass core as the laminate is cooled from the processing temperatures used in laminate manufacture. This shrinkage gives rise to substantial tensile stresses in the plastic surface layers and compressive stresses in the glass core element in the planes parallel to the glass surface.
At the edges of the glass-plastic laminate, the tensile stresses in the surface layers are translated into bending moments which exert a large tensile stress in a direction normal to the glass core layer and across the exposed edge thereof. In the presence of this large tensile stress, referred to as a bond stress, mid-plane or shaling fracture of the glass core layer, which is under planar compression, can be initiated by any surface defects present at the edge of the glass core, resulting in separation of the glass core and the formation of two lens fragments, each comprising one of the plastic surface layers with a section of glass core bonded thereto.
As disclosed in the concurrently filed, commonly assigned copending application of A. A. Spycher, Ser. No. 36,796, the incidence of shaling fracture during the handling of a glass-plastic laminated lens blank can be significantly reduced through the use of a lens blank edge configuration wherein the bond stress exerted by the plastic surface layers is shifted to a point within the body of the glass core element which is spaced away from the edge thereof. However, while such blanks are more durable during lens blank shipment and through the initial stages of blank finishing which may comprise the grinding and polishing of the lens optical surfaces, they are still prone to breakage during edge finishing.
The production of a mounted lens assembly typically comprises an edge finishing step wherein material is removed from part or all of the edge of the lens to shape the lens to a selected configuration for mounting. In the case of glass-plastic laminated lens blanks of the kind herein described, it is found that, during removal of plastic and glass material from the edge of the lens during edge finishing, large flaws are introduced into the edge of the glass core element. In the presence of these flaws, the combination of vibration during edge finishing and the stresses exerted by the plastic surface layers of the laminate frequently results in the shaling failure of the lens blank before the edge finishing process can be completed.
It is a principal object of the present invention to provide an edge finishing method and apparatus which can be used to finish glass-plastic lens blanks comprising a thin compressively stressed glass core member without causing shaling fracture of the laminated lens blank.
It is a further object of the present invention to provide a finished glass-plastic laminated lens which exhibits enhanced resistance to shaling fracture in use.
Further objects and advantages of the invention will become apparent from the following description thereof.